The present invention relates to transformer cores, and more particularly to transformer cores made from strip or ribbon composed of ferromagnetic material, particularly amorphous metal alloys.
Transformers conventionally used in distribution, industrial, power, and dry-type applications are typically of the wound or stack-core variety. Wound core transformers are generally utilized in high volume applications, such as distribution transformers, since the wound core design is conducive to automated, mass production manufacturing techniques. Equipment has been developed to wind a ferromagnetic core strip around and through the window of a pre-formed, multiple turns coil to produce a core and coil assembly. However, the most common manufacturing procedure involves winding or stacking the core independently of the pre-formed coils with which the core will ultimately be linked. The latter arrangement requires that the core be formed with one or more joints for wound core and multiple joints for stack core. Core laminations are separated at those joints to open the core, thereby permitting its insertion into the coil window(s). The core is then closed to remake the joint. This procedure is commonly referred to as xe2x80x9clacingxe2x80x9d the core with a coil.
A typical process for manufacturing a wound core composed of amorphous metal consists of the following steps: ribbon winding, lamination cutting, lamination stacking or lamination winding, annealing, and core edge finishing. The amorphous metal core manufacturing process, including ribbon winding, lamination cutting, lamination stacking, and strip wrapping is described in U.S. Pat. Nos. 5,285,565; 5,327,806; 5,063,654; 5,528,817; 5,329,270; and 5,155,899.
A finished core has a rectangular shape with the joint window in one end yoke. The core legs are rigid and the joint can be opened for coil insertion. Amorphous laminations have a thinness of about 0.001 inch. This causes the core manufacturing process of wound amorphous metal cores to be relatively complex, as compared with manufacture of cores wound from transformer steel material composed of cold rolled grain oriented (SiFe). In grain-oriented silicon steel, not only are the thicknesses of the cold rolled grain-oriented layers substantially thicker (generally in excess of about 0.013 inch), but in addition, the grain-oriented silicon steel is particularly flexible. These combinations of technical features, i.e., greater thicknesses and substantially greater flexibility in silicon steels immediately differentiates the silicon steel from amorphous metal strips, particularly annealed amorphous metal strips and obviates many of the technical problems associated with the handling of amorphous metal strips. The consistency in quality of the process used to form the core from its annulus shape into rectangular shape is greatly dependent on the amorphous metal lamination stack factor, since the joint overlaps need to match properly from one end of the lamination stack factor, since the joint overlaps need to match properly from one end of the lamination to the other end in the xe2x80x98stair-stepxe2x80x99 fashion. If the core forming process is not carried out properly, the core can be over-stressed in the core leg and corner sections during the strip wrapping and core forming processes which will negatively affect the core loss and exciting power properties of the finished core.
Core-coil configurations conventionally used in single phase amorphous metal transformers are: core type, comprising one core, two core limbs, and two coils; shell type, comprising two cores, three core limbs, and one coil. Three phase amorphous metal transformer, generally use core-coil configurations of the following types: four cores, five core limbs, and three coils; three cores, three core limbs, and three coils. In each of these configurations, the cores have to be assembled together to align the limbs and ensure that the coils can be inserted with proper clearances. Depending on the size of the transformer, a matrix of multiple cores of the same sizes can be assembled together for larger kVA sizes. The alignment process of the cores"" limbs for coil insertion can be relatively complex. Furthermore, in aligning the multiple core limbs, the procedure utilized exerts additional stress on the cores as each core limb is flexed and bent into position. This additional stress tends to increase the core loss resulting in the completed transformer.
The core lamination is brittle from the annealing process and requires extra care, time, and special equipment to open and close the core joints in the transformer assembly process. This is an intrinsic property of the annealed amorphous metal and cannot be avoided. Lamination breakage and flaking is not readily avoidable during this process opening and closing the core joint, but ideally is minimized. The presence of flakes can have broadened detriments to the operation of the transformer. Flakes interspersed between laminar layers can reduce the face-to-face contact of the laminations in a wound core, and thus reduce the overall operating efficiency of the transformer. Flakes and the site of a laced joint also reduces the face-to-face contact, reduces the overlap between mating joint sections and again reduces the overall operating efficiency of the transformer. This is particularly important in the locus of the laced joint as it is at this point that the greatest losses are expected to occur due to flaking. Containment methods are required to ensure that the broken flakes do not enter into the coils and create potential short circuit conditions between layers within the core. Stresses induced on the laminations during opening and closing of the core joints oftentimes causes a permanent increase of the core loss and exciting power in the completed transformer, as well as permanent reductions in operating efficiency of the transformer.
Thus, it would be particularly advantageous to provide an amorphous metal core which inherently features a reduced likelihood of lamination breakage which may occur during the assembly of a power transformer.
It would also be particularly advantageous to provide an amorphous metal core which inherently features reduced stress conditions within the wound, and laminated amorphous metal core, particularly three-limbed amorphous metal cores suited for use in three-phase transformers.